The present invention relates to a method and apparatus for generating power utilizing reverse electrodialysis.
As well known, electrical energy may be generated from the free energy of mixing of two ionic solutions by reverse electrodialysis. This technique utilizes a reverse electrodialysis unit including a membrane stack having alternating cation and anion exchange membranes, an electrode at each end of the stack, a first pathway through the reverse electrodialysis unit for a concentrated ionic solution, and a second pathway through the reverse electrodialysis unit for a dilute ionic solution. When the concentrated ionic solution is introduced into the first pathway, and the dilute ionic solution is introduced into the second pathway, solute from the concentrated solution in the first pathway passes through the membranes to the dilute solution in the second pathway, this being accompanied by the generation of an output electrical current and voltage across the electrodes at the ends of the stack. The voltage generated by the concentration difference across each pair of membranes will be very low, even when there is zero load current, but this voltage is multiplied by increasing the number of alternating cation and anion exchange membranes to separate the two solutions in the membrane stack. Further details of this technique may be had from existing publications, such a "Electric Power From Differences In Salinity", Science, Feb. 13, 1976, Vol. 191, pp 557-9, and the references cited therein.
The above described reverse electrodialysis (hereinafter sometimes called "RED") technique appears to have possibilities for producing low cost energy, particularly with highly concentrated brines. However, insofar as we are aware, all investigators to date have considered this technique only in arrangements wherein one or both of the concentrated and dilute solutions to be mixed (hereinafter sometimes called "mixing pairs") must be continually replenished. Thus, such known techniques could be exploited only where located close to large quantities of mixing pairs, such as: Dead Sea brine, fresh water; Dead Sea brine, sea water; and sea water, river water. In addition, such known techniques involve very significant problems in the transport of the mixing pairs to the plant, and in the disposal of the mixed solution (hereinafter sometimes called "spent brine") exiting from the RED unit.